1. Field of the Invention
The present invention relates to a substrate processing apparatus used when a substrate such as a semiconductor wafer is subjected to a predetermined process such as a cleaning process, and a technique of controlling the inert gas concentration of a processing liquid.
2. Description of the Background Art
In a substrate cleaning process performed in the manufacturing process of a semiconductor device and the like, pure water, or a mixed liquid of pure water and hydrogen peroxide, ammonium etc. is used as a processing liquid. For example, a megasonic cleaning apparatus employs in some times such a processing liquid that is obtained by dissolving nitrogen gas, as an inert gas, in pure water under atmospheric pressure so as to have a concentration of approximately from 5 ppm to 20 ppm, in order to increase the efficiency of cleaning.
FIG. 13 is a diagram showing a conventional substrate processing apparatus 1000. The apparatus 1000 comprises mainly a pure water adjusting part 1001 and a processing part 1012.
The pure water adjusting part 1001 adjusts the nitrogen gas concentration of pure water to a predetermined concentration and then supplies it to the processing part 1012 that performs, for example, megasonic cleaning. The pure water so adjusted is obtained by dissolving nitrogen gas that is supplied from a nitrogen supply source 1004 provided as factory utility and the like via a nitrogen gas supply path 1005, in pure water that is supplied from a pure water supply source 1002 provided as factory utility and the like via a pure water supply path 1003.
The pure water adjusting part 1001 comprises a dissolving part 1006 for dissolving nitrogen in pure water supplied from the pure water supply source 1002, a nitrogen concentration meter 1007 for measuring the nitrogen gas concentration of the pure water passing through the dissolving part 1006, a pressure gauge 1008 for measuring the pressure of nitrogen gas supplied through the nitrogen gas supply path 1005, a flow meter 1009 for measuring the flow of nitrogen gas, a valve 1010, and a control part 1011 for controlling the opening and closing of the valve 1010, based on measurements of the nitrogen concentration meter 1007.
FIG. 14 is a diagram showing a sequence of operating steps for dissolving nitrogen gas in the pure water adjusting part 1001, such that the nitrogen gas concentration of pure water has a predetermined target concentration C. As shown in FIG. 13, in the pure water adjusting part 1001, firstly, the supply of nitrogen gas to the dissolving part 1006 is stopped, and an initial nitrogen gas concentration C0, corresponding to the nitrogen gas concentration in the state that no nitrogen gas is dissolved in pure water, (i.e., blank data) is taken by the nitrogen concentration meter 1007 (step S1001). Subsequently, a required dissolution concentration ΔC is found by subtracting the obtained initial nitrogen gas concentration C0 from the target concentration C (step S1002). A target gas pressure Pt to the required dissolution concentration ΔC is found from a chart indicating the relationship between pressure and dissolved concentration (step S1003). Next, a gas flow Ft to the target gas pressure Pt is found from a chart indicating the relationship between pressure and flow (step S1004), and a gas pressure rising time T to the target gas pressure Pt is found from a chart indicating the relationship between pressure and pressure rising time (step S1005).
Thus, the initial setting value is determined, and a nitrogen dissolving process based on this value is started (step S1006). Thereafter, the nitrogen concentration meter 1007 sequentially measures nitrogen gas concentration, and the gas flow and gas pressure are controlled proportionally based on the measured value (step S1007). This enables to supply pure water in which nitrogen gas of a target concentration is dissolved.
Such nitrogen dissolving process is disclosed, for example, in Japanese Patent Application Laid-Open No. 8-196879.
However, the nitrogen dissolving process in the conventional substrate processing apparatus suffers from the following problem.
Specifically, the initial nitrogen gas concentration C0 in the pure water supplied to the dissolving part 1006 can vary depending on the pure water manufacturing circumstances and storage environment in the pure water supply source 1002. When the supplied pure water has a higher nitrogen gas concentration than the target concentration C, the conventional method fails to reduce this concentration due to lack of the function of degassing nitrogen gas in the dissolving part 1006. In this case, it is therefore unavoidable that pure water having a higher nitrogen gas concentration than the target concentration C is supplied to the processing part 1012.
For example, when performing a megasonic cleaning in the processing part 1012, even with the same input electric power, the physical energy intensity exerted on the substrate varies depending on the nitrogen gas concentration. Since this variation affects the effect of cleaning a substrate, the use of pure water having a higher nitrogen gas concentration than the target concentration C lowers the percentage of removal of particles PRE or damages substrates to be processed in the processing part 1012, resulting in poor manufacturing yield.